MECHANISM OF THE KNEE-JOINT. 239 



distant point in one of the bones when in motion, or of the 

 point of a rod prolonged from it. I was enabled leisurely to 

 examine the path pursued by that point during flexion and 

 extension as projected on a plane parallel to the transverse 

 plane of the joint. The path described by the point of the 

 rod was found to be the same in both movements, but pre- 

 sented a f<jrm which cannot be referred to the movements of 

 the joint as hithei-to conceived- It was a continuous curve, 

 which crossed that perpendicular line in the field of view, 

 with which the point of the rod in the extended position was 

 coincident, before it again came to rest in it at the close of 

 flexion. Supposing the leg to be fixed perpendicularly, what- 

 ever the nature of the profile curve described by the point of 

 the rod fixed in the axis of the thigh-bone during flexion and 

 extension — whether cycloidal, if the profile curve of the femoral 

 condyles be circidar, or some form of equiangular spiral, if 

 the condyles possess that character, it appeared evident that 

 the line projected on a plane parallel to the transverse plane 

 of the joint must be a perpendicular line. It also appeared 

 evident that if Meyer's rotation really occurred, the line pro- 

 jected on this transverse plane would be curved above and 

 pass below into a straight line. I fountl, however, on viewing 

 the point of the rod as projected in its course on this plane, 

 that the line was a continuous curve, concave throughout 

 towards the inner side in both the right and left lines. 



It was evident, therefore, in the fii-st place, that the upper 

 part of the curve was due to Meyer's rotation, renewed in 

 consequence of the obliquity of the shaft of the thigh-bone ; 

 and in tlie second place that the external condyle, and the 

 two posterior thirds of the internal do not roll and glide in 

 parallel pliin(\>^, but along curved paths. But as the inner 

 condyle is rolling while pa.ssing round Meyer's curve, it is 

 clear that from first to last, in tlexion and extension, the 

 condyles of the femur roll and glide along curved j)aths. 



